1. Field of the Invention
The present invention relates to a method of and an apparatus for processing internal combustion engine speed data. Such a method and apparatus may be used, for instance, to detect engine misfire and indicate the degree of road surface roughness.
2. Discussion of Prior Art
A known technique for detecting misfire of an internal combustion engine measures engine crankshaft speed variations and compares the variations with one or more thresholds. For instance, a respective fixed threshold may be used for each of a plurality of engine speed ranges, for instance 512 revolutions per minute (RPM) wide. Thus, for the prevailing engine speed, each speed variation is compared with the respective threshold and, if the threshold is exceeded, this is taken to indicate an engine misfire.
Setting a value of each threshold involves a compromise between detecting excessive numbers of false misfires and failing to detect genuine misfires. Engine crankshaft speed is subject to speed variations caused by random noise, for instance as a vehicle driven by the engine moves over a rough road or other surface or caused by drive line vibration in a transmission connected to the engine. Also, because of manufacturing and other tolerances, different engines of the same design can exhibit different amplitudes of speed variations. Further, the aging effects of engines can alter the amplitude of speed variations. If the threshold were set too low, then all of these effects could result in excessive false detections of misfire i.e. detection of misfire when the engine did not misfire.
In order to avoid excessive false detection of misfire, the threshold must be set high enough so as to make detection substantially immune from these effects. However, if the threshold is set to too high a value, then this technique may fail to detect substantial numbers of engine misfires. Although, in theory, each engine could be tested by deliberately introducing a misfire and setting the or each threshold so as to provide an acceptable detection performance, this is impractical for commercial production of engines and, in any event, cannot take into account various of the effects described above, such as engine aging and road surface roughness.